CRISPR-like ‘immune’ system discovered in giant virus

Ewen Callaway in Nature:

Gigantic mimiviruses fend off invaders using defences similar to the CRISPR system deployed by bacteria and other microorganisms, French researchers report¹.

…Like prokaryotes, mimiviruses are plagued by viruses known as virophages, Raoult, La Scola and their colleagues reported in 2008². Six years later, in 2014, they found a virophage — named Zamilon — that infects some kinds of mimivirus but not others³. Raoult hypothesized that these infections, which sap a mimivirus’s capacity to copy itself, could have led to the evolution of a defence system much like CRISPR. In bacteria and another kind of prokaryote, called archaea, CRISPR systems store a library of short DNA sequences that match those of phages and other invading DNA. When a foreign DNA sequence with matching sequences in this library attacks a cell, specialized ‘Cas’ enzymes unwind the intruder DNA and chop it into pieces, stopping an infection. Biologists have now repurposed CRISPR as a technology to edit genomes. To determine whether mimiviruses have a similar defence system, Raoult’s team analysed the genomes of 60 mimivirus strains and looked for sequences that match those of the Zamilon virophage. Mimiviruses that were resistant to Zamilon also harboured a short stretch of DNA that matched that of the phage. Adjacent to these sequences, Raoult’s team found genes encoding enzymes that can degrade and unwind DNA. In CRISPR immunity, too, the genes encoding the Cas enzymes sit beside the sequences that recognize the virus. Blocking activity of different components of the system made the mimiviruses susceptible to Zamilon virophage attack.

More here.